That was way too much trouble for me, so I installed "Add-on Compatibility Reporter".
For example, I'm still using "Smart Bookmarks Bar" plugin with FF6
If my memory serves me correctly, that plugin was not even supported earlier (v4?) version of FF.
After disabling compatibility check, I realized that many old plugins still work. Even without cumbersome file edits.

˙ǝɐʇɐboɹ ǝssǝ uǝɯɐʇ ʇuǝpnɐb 'ʇuɐbǝu ǝnbɐnb 'ʇuɐp ǝɐnb

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They may work but from working with many extensions that do not work even with this compatbility, my way is to do both file edit and compatibility addon.
It really takes a few minutes which is worthwhile especially as firefox has now become great and worthwhile.
File edit is best done in ver 3 as extensions edit are in folders and the manipulation can be done in notepad without the problem of machine language. Starting from Ver 4 onwards gives machine language for many extensions.

FYI, after seeing nightly go to 9, I have changed the max version from 9 to 11. Should give me a month or two of relief.
FF can be quirky and I have found doing the same process leading to 2 different results.

For me, addon compatbility extension is as essential as file edits, which are not cumbersome at all. Takes only 2 mins for all 7-8 extensions that I need to make work in newer FF versions.

Windows 10, Core i7 6850k, ASUS GTX 970

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